Mate you can’t just take random Internet comments like this as medical advice. You pet needs medical attention from an actual vet who can examine the patient.

If you can’t afford that, ask for donations to help you pay the vet or ask local animal shelters or vets if they have any options to help you. But stuff like giving the cat random ointments or changing their foods without any actual diagnosis is not a way to help a suffering animal.

We are taking the jacket off a couple times a week to let the skin air out and so that she can clean herself naturally, but this one spot is horrendous

The fact that you have your cat in a jacket most of the week preventing her from cleaning herself several times a day as is normal for cats is a whole 'nother red flag. That is extreme and not something you should be subjecting a cat to without a vet telling you to do it anyway.

I dunno, those buttholes get cleaned pretty thoroughly multiple times a day. Probably more thoroughly than most human buttholes. Might actually be cleaner than the window sill.